The invention relates to a road marking machine having at least one spray gun for a pumpable marking substance, such as paint, and a combination of two displacement pumps for dosably supplying the marking substance to the spray gun, wherein the combination of the two displacement pumps is driven in proportion with the traveling speed of the road marking machine.
Such a road marking machine is known from German patent application DE 30 07 116 C2. A serious disadvantage of that machine is the branched guiding of the paint streams after leaving the supply chambers of the pump, with the stream guided by valves. The 3/2-port-valves must be positively controlled in that their functional principle is for dividing the interrupted paint streams discharged from the supply chambers into a pulse free paint stream for the using units, and an interrupted remaining paint stream to be recycled to the reservoir. These valves are subject to high wear, in particular with abrasive paints. Furthermore, the valves are complicated and expensive with increasing operating pressures.
Besides the disadvantage associated with recycling a part of the paint stream to the reservoir, it is an additional disadvantage that this part paint stream has to be guided through a throttle, and the resistance of the throttle has to be adjusted to the resistance of the spray gun. Positively controlled valves for the control of paint streams are subject to high wear, in particular when the paints contain solid particles, and are complicated and expensive for high pressures. The same is true for the throttle.
It is a further disadvantage that the pump has to be driven even when the marking lines are interrupted, and that during the interruption the paint stream dedicated for the spray gun has to be recycled back to the reservoir through a valve to be opened and a throttle adjusted to the resistance of the spray gun.
This constant pumping is necessary to have the required spray pressure ready when the spray gun is opened again when a new part of the marking line is started.
The branched guidance of the paint with many paint contacting members is also a disadvantage when the unit has to be designed in stainless steel for paints being aggressive for normal steel, which again will greatly increase the manufacturing costs.
The process known from the patent document cited above has the further disadvantage that the two displacement members are mechanically driven through cam plates. The economic transfer of forces in this manner is limited for the sizes of the utilized pumps. Pumps of this type have been known only for the low pressure range with spray pressures up to 15 bar. For the high pressure range with pressures up to 200 bar, such a mechanical drive would be very heavy, very complicated, and costly for the higher forces involved.